The Linux Programming Interface

1386 Chapter 64

this argument o. Use of this argument is a convenience for certain interac-

tive programs (e.g., script(1)) that use a pseudoterminal and need to set the

attributes of the slave device to be the same as those of the terminal under

which the program is run.

–If the slaveWS argument is non-NULL, perform an ioctl() TIOCSWINSZ operation

to set the window size of the pseudoterminal slave to the values in the

winsize structure pointed to by this argument a. This step is performed for

the same reason as the previous step.

–Use dup2() to duplicate the slave file descriptor to be the standard input,

output, and error for the child s. At this point, the child can now exec an

arbitrary program, and that program can use the standard file descriptors

to communicate with the pseudoterminal. The execed program can per-

form all of the usual terminal-oriented operations that can be performed

by a program running on a conventional terminal.

As with fork(), ptyFork() returns the process ID of the child in the parent process, 0

in the child process, or –1 on error.

Eventually, the child process created by ptyFork() will terminate. If the parent

doesn’t terminate at the same time, then it must wait on the child to eliminate the

resulting zombie. However, this step can often be eliminated, since applications

that employ pseudoterminals are commonly designed so that the parent does ter-

minate at the same time as the child.

BSD derivatives provide two related, nonstandard functions for working with

pseudoterminals. The first of these is openpty(), which opens a pseudoterminal

pair, returns the file descriptors for the master and slave, optionally returns

the name of the slave device, and optionally sets the terminal attributes and

window size from arguments analogous to slaveTermios and slaveWS. The other

function, forkpty(), is the same as our ptyFork(), except that it doesn’t provide

an analog of the snLen argument. On Linux, both of these functions are pro-

vided by glibc and are documented in the openpty(3) manual page.

Listing 64-2: Implementation of ptyFork()

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––pty/pty_fork.c

#include <fcntl.h>

#include <termios.h>

#include <sys/ioctl.h>

#include "pty_master_open.h"

#include "pty_fork.h" /* Declares ptyFork() */

#include "tlpi_hdr.h"

#define MAX_SNAME 1000

pid_t

ptyFork(int *masterFd, char *slaveName, size_t snLen,

const struct termios *slaveTermios, const struct winsize *slaveWS)

{

int mfd, slaveFd, savedErrno;

pid_t childPid;

char slname[MAX_SNAME];